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Self-Organized Nanostructuring in Functional Thin Film Materials

Final Report Summary - FUNMAT (Self-Organized Nanostructuring in Functional Thin Film Materials)

I consider my work on fullerene-like carbon-based thin solid films as the most exciting in terms of defining a new class of mechanically resilient compounds with tuneable surface energy (water or oil repelling vs wetting properties). Some people call the material rubber-diamond because of its combination of seemingly contradictory properties. We thus discovered fullerene-like phosphorus carbide (FL-CPx) and FL-CFx (like a super-Teflon). This family of materials are among the most resilient materials, counting also FL-CNx (carbon nitride) discovered in my group before the start of this ERC project. I have stimulated other groups to enter the field (William & Mary; Budapest; Newcastle; Dayton).

Of similar interest is my work reporting the first description of multifunctional nanolaminated “MAX” phase epitaxial films (Ti2AlN and Ti3SiC2). Novel phases of Ti4SiC3 and Ti4GeC3 were discovered. MAX-film research has now spread to Uppsala, Urbana, Aachen, Rossendorf, Lyon, and more. MAX derivatives nc-TiC/a-SiC nanocomposites were also demonstrated by us as a wear-resistant electrically conducting contact material to replace gold.

With the ERC project I have advanced the concept developed at Linköping that metastable Ti1-xAlxN films age harden by spinodal decomposition into coherent cubic-phase nm-size domains. It concerns smart materials that self-organise into intricate nano structures by phase transformations called spinodal decomposition. This has inspired other groups (incl. Aachen, Leoben, Illinois) to enter the field. The great usefulness is that the material actually gets harder during heating as from the friction during a coated cutting tool operation. The nanostructure formation was investigated by the most advanced electron and ion microscopy available in the world.

I have added conceptually new understanding to the surperhard nanocomposites, starting out with the archetype nc-TiN/a-Si3N4. Microscopical observations supported by ab initio calculations revealed the epitaxial stabilization of cubic-SiNx on TiN(001). This work makes an excellent connection to my earlier studies of the mechanical properties of nitrides, both as superlattices and in terms of their dislocation dynamics.

We ventured into the unexplored compositional field of the Sc-Al-N system and grew the first single-crystal inverse perovskite (ip)-Sc3AlN with the a lattice parameter 4.40 Å, hardness 14.2 GPa, elastic modulus 249 GPa, and RT resistivity 41.2 μΩcm. Inspired by this discovery, we reported on the mechanical and thermodynamic stability of isoelectronic ip-Sc3EN (E=B,Al, Ga,In). We confirm energy and dynamic stability of the recently synthesized cubic phases Sc3AlN and Sc3InN, and predict stability of ip-Sc3GaN and metastability of triclinic-Sc3BN. More phases and reaction paths discovered in my group include:

• YxAl1-xN solid solution thin films in the miscibility gap
• 2-D nanolabyrinthine intergrown non-isostructural c-ZrN/w-AlN: {110}║{11-20} interfaces
• B3-like Si-N phases in superhard TiN-SiNx nanocomposites with Si tetrahedrally coordinated by N between B1-TiN(001) is dynamically and thermodynamically stable to Si vacancies.
• fcc-(Al1−xCrx)2O3[0.6≤x≤0.7] by stabilization from 1/3 metal vacancies ao=4.04Å; E~225 GPa
• Ti7Si2C5 (c=60.2 Å) grown on Al2O3 by magnetron sputter epitaxy
• Nb2GeC MAX phase as the first 211 MAX phase discovered since the 1960s.
• The bcc-NiSi phase (space group I-43m 217, a=2.72 Å) by topotactic reaction in Ni/Si diffusion couples at 150-350, °C. bcc-NiSi transforms to NiSi2 with Ea ~0.6 eV
• Step-flow growth of Ti3SiC2(0001) with {11-2 0}&{1-1 00} terrace step by AFM and HIM

Our most recent achievement is the determination of stable adsorption sites, diffusion pathways, and migration rates of N adatoms, together with reaction pathways leading to the associative formation and desorption of N2 molecules on TiN(001). We perform both ab-initio and classical molecular dynamics (AIMD and CMD) simulations as a function of temperature T, in which interatomic forces are obtained from DFT and the modified embedded atom method (MEAM) potential optimized for TiN bulk and surface properties. As another example of the power of MD studies, results by us predict that Ti adatoms and TiN2 admolecules are the most mobile species on TiN(001) terraces. Both adspecies are rapidly incorporated at (001) island descending steps, hence contribute to layer-by-layer growth.